1. Field of the Invention
The present invention relates to a catalyst for purification of automobile exhaust gas, and a method for manufacturing the same.
2. Related Background Art
Various catalysts have so far been used to sufficiently purify components such as carbon monoxide (CO), hydrocarbon (HC), and nitrogen oxide (NOx) contained in exhaust gas emitted from an internal combustion engine of an automobile. As such a catalyst for purification of automobile exhaust gas, a catalyst is known in which a noble metal such as platinum (Pt) or rhodium (Rh) is supported on a support made of γ-Al2O3 or the like, for example. Meanwhile, as a method for manufacturing such a catalyst for purification of automobile exhaust gas, a method has generally been employed with which a noble metal is supported on a support by supporting a solution containing a salt of the noble metal on the support and calcining those. In addition, studies on various methods for manufacturing a catalyst for purification of automobile exhaust gas have been conducted in recent years to obtain a catalyst having a higher catalytic activity.
For example, Japanese Unexamined Patent Application Publication No. 2006-55807 (Document 1) discloses a method for manufacturing a catalyst. In this method, a catalyst having noble metal clusters supported therein is obtained by depositing a polynuclear complex comprising a plurality of organic polydentate ligands and a plurality of noble metal atoms on an oxide support, and then removing the organic polydentate ligands. Meanwhile, Japanese Unexamined Patent Application Publication No. 2007-222806 (Document 2) discloses a method for manufacturing a noble metal catalyst. This method involves the formation of a gelled product by introducing a platinum complex such as platinum acetylacetonate or dinitro diamine platinum into an aqueous solution of boehmite. However, it is not necessarily possible to obtain a catalyst having a sufficient catalytic activity by the methods for manufacturing a catalyst described in Documents 1 and 2.
In addition, Japanese Unexamined Patent Application Publication No. 2008-13533 (Document 3) discloses a method for obtaining a catalyst by supporting the multiple-complex-containing compound on the porous support by means of impregnating. The method described in Document 3 can be implemented when Pt is used as the nuclear (metal) in the complex. When, however, Rh is used as the nuclear (metal) of the complex, multiple-complex-containing compound itself cannot be manufactured under the manufacturing conditions described in the Document. Accordingly, a catalyst having Rh supported therein cannot be manufactured on the basis of the method for obtaining a catalyst described Document 3.
In addition, Japanese Unexamined Patent Application Publication No. 2007-230924 (Document 4) describes a method for obtaining a catalyst. In this method, the catalyst is obtained by impregnating a support with a solution containing a metal complex, and calcining after drying those. In the metal complex, ligands are coordinated to one metal atom or a plurality of metal atoms of the same kind, and at least one of the ligands has a carboxyl group that is not coordinated to the metal atom or the like (for example, a dicarboxylic acid ligand). The method described in Document 4 can be implemented when Pt is used as the metal atom. When, however, Rh is used as the metal atom, the metal complex itself cannot be manufactured under manufacturing conditions described in the Document. Accordingly, a catalyst having Rh supported therein cannot be manufactured on the basis of the method for obtaining a catalyst described in Document 4.
Moreover, Japanese Unexamined Patent Application Publication No. 2007-229642 (Document 5) describes a method for obtaining a catalyst by impregnating a support, to which a compound having a coordinatable functional group is bonded, with a solution that contains a metal complex, and by calcining after drying those. However, in the method described in Document 5, when a rhodium carboxylate such as [Rh2(CH3COO2)4] or [Rh2(C6H5COO2)4], which is a metal complex described in Document 5, is used, not only rhodium cannot be supported sufficiently, but also a catalyst having a sufficient catalytic function cannot necessarily be obtained.